Because previous studies attempting to judge the quality of prescribing by general practitioners had been based on unrepresentative groups of doctors and had employed idiosyncratic criteria, which did not adequately take into account the views of general practitioners themselves, prescriptions issued during a single month by forty-six randomly selected general practitioners in the Grampian region were analysed in detail and a qualitative measurement of the prescribing of 'undesirable' drugs applied. This measurement was derived from the general practitioners' own opinions about a systematic selection of proprietary drugs, obtained from a separate questionnaire study. Factors associated with quantitative and qualitative variations in prescribing were identified and although there was no general correspondence between costs and qualitative measurements, it was possible to typify, on the basis of cost, four groups of prescribers with distinct characteristics. In particular, partnership size and duration of qualification of the doctor were confirmed to be of some discriminatory value, and membership of the Royal College of General Practitioners and/or possession of a teaching appointment appeared to dispose toward 'better' prescribing. The general practitioners' opinions about drugs, as obtained from the questionnaire, corresponded with their actual prescribing. There were important differences of opinion between general practitioners and hospital physicians about certain drugs in the questionnaire and these distinctions, and the close correspondence of attitudes with behaviour, have implications for continuing education. Conversely, thehigh level of agreement amongst doctors in both general and hospital practice concerning the highly undesirable nature of certain drugs should further encourage the voluntary limitation of prescription of amphetamines, barbiturates, methaqualone and stimulant appetite suppressant drugs, in particular.